The invention relates to a method of manufacturing fluorine-doped optical fibers.
The use of fluorine as a refractive index decreasing dopant in the manufacture of optical fibers on the basis of fused silica is known from the following patents and publications:
(1) A. Muhlich, K. Rau, F. Simmat, N. Treber, 1st ECOC, IEE, London 1975 PA0 (2) K. Abe, 2nd ECOC, IEE, Paris 1976 PA0 (3) DE-PS No. 25 38 313 (corresponding to U.S. Pat. No. 4,045,198) PA0 (4) D. Kuppers, J. Koenings, H. Wilson, 3rd ECOC, Munich 1977 PA0 (5) D. Kuppers, J. Koenings, H. Wilson, J. Electrochem. Soc. 125 (1978) 1298 PA0 (6) A. Muhlich, K. Rau, N. Treber, 3rd ECOC, Munich 1977 PA0 (7) K. Rau, A. Muhlich, N. Treber, Topical Meeting on Fiber Transmission, IEEE, Williamsburg 1977 PA0 (8) DE-OS No. 29 31 092 (corresponding to U.S. Pat. No. 4,221,825) PA0 (9) B. J. Ainslie, C. R. Day, P. W. France, K. J. Beales, G. R. Newns, Electron. Lett. 15 (1979) 411 PA0 (10) J. W. Fleming, V. R. Raju, Electron. Lett. 17 (1981) 867.
In the manufacture of fused silica optical fibers according to both the thermally activated MCVD method (publications 2, 3, 9) and in plasma-activated manufacturing processes (publications 4, 5, 6, 7, 10) fluorine may be used as a dopant. The following compounds may serve as fluorine sources: SiF.sub.4 (2, 4, 5), NF.sub.3, SF.sub.6 (8), CCl.sub.2 F.sub.2 (3,8) and CF.sub.4 (2, 9). By using the MCVD method it is possible to produce glass layers with refractive index differences of approximately 0.5% by fluorine doping (2, 3). However, relatively large quantities of fluorine compound have to be consumed to reach these values. According to (2), a difference in refractive index of 0.5% is reached only with a SiF.sub.4 /SiCl.sub.4 ratio of 12:1. The large excess of SiF.sub.4, however, degrades the deposition from the gas phase. Therefore, in the MCVD process fluorine is usually used only together with other dopants (9, 10).
The use of plasma activation under normal pressure (6, 8) and at low pressure (4, 5) permits the production of glass layers with differences in refractive index of 1% (7) and 1.3% (5), respectively. In these multimode optical fibers doped with fluorine only, optical attenuations of 2.2 dB/km at 1060 nm were realized. Monomode optical fibers have so far not been manufactured in this manner. A high fluorine compound concentration, as compared to the SiCl.sub.4, in the gaseous phase is also required in this process.